Developments in wireless technology typically require wireless operators to deploy new antenna equipment in their networks. Disadvantageously, towers have become cluttered with multiple antennas while installation and maintenance have become more complicated. Basestation antennas typically covered a single narrow band. This has resulted in a plethora of antennas being installed at a site. Local governments have imposed restrictions and made getting approval for new sites difficult due to the visual pollution of so many antennas. Some antenna designs have attempted to combine two bands and extend bandwidth, but still many antennas are required due to the proliferation of many air-interface standards and bands.
Cellular basestation antennas generally radiate dual-slant polarization inclined at +/−45° to vertical. However, in a dual band dual polarization antenna where the radiating elements associated with a low frequency band and a high frequency band must be interspersed, it may be desirable to have the radiators of one band, usually the high frequency band inclined so that those radiators radiate dual slant polarization and the radiators of the second band, usually the low frequency band, arranged to radiate vertical and horizontal polarization. This avoids obstruction of the radiating elements of one band by the radiating elements of the other band.
Although the radiators of one band may be aligned to radiate vertical and horizontal polarization, both bands generally radiate dual-slant polarization. An equal-split 180° hybrid is required to effect this transformation.
An equal-split 180° hybrid coupler or junction (simply “hybrid” hereinafter) is a well-known four-port directional coupler designed for a 3 dB power split (i.e., an equal power split). For example, a rat-race coupler is such a 180° hybrid. The 180° hybrid has two input and two output ports. One input port is typically referred to as the Sum input (designated by sigma, Σ) and the other input is typically referred to as the Difference input (designated by delta, Δ). A signal input to the Σ input port of the 180° hybrid produces the signal split at the output ports both in phase. However, if the signal is input to the Δ input port, the 180° hybrid produces the signal split at the output ports, one in phase and the other 180° out of phase. A rat-race 180° hybrid has four ports, adjacent ports being separated by a section of metal tracks (e.g., microstrip or stripline) or waveguide. Three sections between the four ports (port 1 to port 2, port 2 to port 3, port 3 to port 4) are one quarter wavelength (λ/4) apart. The first and last ports (port 1 to port 4) are separated by a section of three quarters wavelength (3λ/4). Disadvantageously, such a 180° hybrid coupler is narrowband, only giving a correct phase at one frequency.